1. Field of the Invention
The invention relates to a turbomachine rotor assembly, more particularly such an assembly wherein the stress concentrations between the blade root and the rotor disk are reduced.
2. Description of the Prior Art
Many ways of attaching turbine rotor blades to a turbine rotor disk have been proposed in the many years turbomachines have been in use. Typically, the turbine blades have been formed with a root having a specific cross sectional shape, such as a dovetail or "fir-tree", which is inserted into a correspondingly shaped groove formed in the turbine rotor disk. The groove may be formed extending circumferentially about the rotor disk, or a plurality of axially extending grooves may be formed about the periphery.
The blade root and the rotor disk groove have interlocking surfaces so as to prevent the radial movement of the turbine blades with respect to the rotor disk during operation of the turbomachine. Ideally, the blade root and the rotor disk groove have minimal clearances to prevent any vibration or unnecessary motion between the blade and the disk. Also, the control bearing surfaces should be absolutely parallel to each other to ensure the maximum bearing area so as to minimize stress concentration.
As a practical matter, however, there must be adequate tolerances between the blade root and the grooves in the rotor disk in order to facilitate the assembly of these elements. Also, the surfaces formed on the blade root and those corresponding surfaces formed on the rotor disk invariably have surface blemishes and a degree of non-parallelism which serves to concentrate the stresses in those areas where contact between these surfaces takes place.
Devices have been proposed to apply a pre-load force between the turbine blade root and the rotor disk in order to take up the clearances between the interengaging surfaces. Although these devices have been successful, they have not alleviated the problems generated by the non-parallelism of the surfaces, the surface defects, or other factors which create increased stress concentrations in the contact areas between the surfaces.
In the field of high-temperature gas turbines, it has been proposed to incorporate a compliant layer of material between the blade root and the rotor disk groove to minimize stresses imparted to brittle, ceramic blade roots. These typically have included placing a metallic felt layer between the surfaces, forming the entire "fir tree" root from a compliant material, or forming compliant areas on the sides of the rotor disk groove by machining methods. All of the known devices have served to increase both the cost and complexity of manufacturing the turbo machine and, consequently, have not acheived an ideal solution to the problem.
It is also known to form superplastic metallic articles, including a fiber reinforced structure, by electrodeposition or electroforming processes.